FIG. 1 illustrates an integrated circuit that is surface mounted over a circuit board. This integrated circuit includes a silicon (Si) circuit 101. An insulating film 102 coats the underside of silicon circuit 101. An epoxy layer 103 and a silicon cap 104 cover silicon circuit 101. A metal bridge 105 electrically connects silicon circuit 101 to silicon post 106. Metal bridge 105 and silicon post 106 provide for a contact lead for the integrated circuit. Epoxy layer 103 and silicon cap 104 also cover metal bridge 105. Epoxy layer 103 separates silicon circuit 101 and silicon post 106.
A nickel (Ni) plate contact layer 107 covers silicon post 106 and forms a butt-joint with metal bridge 105. Contact layer 107 is electrically coupled to silicon post 106 and metal bridge 105. Contact layer 107 provides the integrated circuit with a connection point to external circuitry.
As illustrated in FIG. 1, the contact lead of the integrated circuit has been soldered to circuit board conductor 109 with solder fillet 108. Circuit board conductor 109 has been formed over circuit board substrate 110.
The contact lead for the integrated circuit illustrated in FIG. 1 provides for various advantages. For example, contact layer 107 covers the sidewalls of silicon post 106, which helps to strengthen the bonding between the integrated circuit and the circuit board. This is due to the fact that solder can be placed on contact layer 107 on the sidewalls of silicon post 106 as illustrated in FIG. 1. It also facilitates inspection during surface mount of the integrated circuit to the circuit board. Whether a good mount is made can be easily confirmed by seeing the solder on the sidewalls of silicon post 106.
Furthermore, contact layer 107 extends over the sidewalls of silicon post 106 and contacts the side of metal bridge 105, forming a butt-joint interface between contact layer 107 and metal bridge 105. This provides for an electrical contact between circuit board conductor 109 and silicon circuit 101.
The butt-joint interface of the integrated circuit contact lead of FIG. 1, however, cannot be formed with much certainty or control over its resulting reliability or bonding adhesion between contact layer 107 and metal bridge 105. There are a number of reasons for this. The physical surface of the side of metal bridge 105 might not be fiat enough to ensure a reliable bond at this butt-joint interface. Furthermore, the side of metal bridge 105 is difficult to clean because of its location on the side of the wafer. The bond at this butt-joint interface therefore may be weakened if the side of metal bridge 105 has not been thoroughly cleaned. The formation of a butt-joint interface also limits the materials that may be used for contact layer 107 and metal bridge 105. This is so because metal bridge 105 will typically comprise more than one metal layer. The bonding layer of contact layer 107 then has to be formed so as to bond with each metal layer at the side of metal bridge 105 in order to form an effective contact. Accordingly, the selection of materials that may be used for metal bridge 105 and for the bonding layer of contact layer 107 is limited.